It has been known for some time that merely looking at a plume from a smokestack will not yield reliable information on the emission of pollutants. In fact, a stack that appears to be emitting little or no material may in fact be producing great quantities of particulate material in respect to a stack that appears to be emitting copiously. This is due primarily to the size of the emitted particles. If the emitted particles are of a size that interact strongly with visible radiant energy, a relatively small total mass will produce a large optical effect. Also, very small, transparent particles will have much less optical effect than their total mass would lead one to expect. Accordingly, visible radiant energy has proven very unsatisfactory in evaluating emissions.
In general, two methods of measuring particulate effluents have been developed. In the optical method the typical system measures the opacity of a specified path for a specified optical band. In the mechanical method a series of mechanical filters and particle collectors are subjected to the effluent for a period of time. The collectors are carefully weighed before and after and the weight gain noted. The total weight gain is related to the total emission. The plot of weight gain as a function of filter size will yield a particle size distribution, which information is very useful in monitoring a particular process or system.
The mechanical measurements are very time consuming and do not read in real time. Also the readings are on an average value taken over the particle collection period.
The optical systems, while reading in real time and being responsive to rapid fluctuations in emissions, do not yield data on particle size distribution.